Current control and indicator for high voltage accelerators



Marsh 34, 1967 c. c. THOMPSON, JR 3,309,616

CURRENT CONTROL AND INDICATOR FOR HIGH VOLTAGE ACGELERATORS Filed Dec. 17, 1963 INVENTOR lilllll Ilill i |lzi wwwiuoumzam 15 W. 35 Cum ua .1 m wwficm \V [.11 p mg 92 I L m m J WIM- CHESTERC THOMPSOMAK ATTORNEYS United States Patent 3,309,616 CURRENT CQNTROL AND INDICATOR FOR HIGH VOLTAGE ACCELERATORS Chester C. Thompson, Jr., Roslyn Heights, N.Y., assignor to Radiation Dynamics, Inc., Westbury, N.Y., a corporation of New York Filed Dec. 17, 1963, Ser. No. 331,243

7 Claims. (Cl. 328-258) The present invention relates to current control and indicating apparatus and more particularly, to a current control and indicating apparatus for a high-voltage linear electron or ion beam accelerator.

The apparatus of the present invention may be employed with conventional electron or ion beam accelerators such as the Van De Graaf generator but, for purposes of explanation, is described as applied to an electron or ion beam generator and accelerator and disclosed in US. Patent No. 2,875,394 in the name of Marshall R.

Cieland. In the Cleland electron and ion beam acceleration apparatus, there is provided a beam tube having a plurality of hollow insulating cylinders which alternate with a plurality of thin, centrally apertured, metallic discs. The entire tube structure is sealed so that a vacuum may be maintained therein and a high voltage, for instance, of the order of one million or more volts is maintained between the high voltage and the low voltage ends of the tube. More particularly, a high voltage is distributed equally between the various metallic discs so that a uniform voltage gradient is established from the high voltage end to the low voltage end of the tube. An electron gun assembly or ion source is located at the high voltage end of the tube and in the electron embodiment comprises a heated cathode and a current control grid, the grid being employed to determine the quantity of electrons emitted by the heated cathode which enter the acceleration portion of the tube. The low voltage end of the tube; that is, the last metallic disc in the tube structure, is usually maintained at ground potential. The beam passing through the aperture in the last disc is extracted from the tube through a suitable electron or ion permeable window and issues into the atmosphere where it may be employed for various irradiation purposes. The tube is evacuated and is housed in a pressurized container because of the particular type of voltage source provided by the aforesaid Cleland patent.

In attempting to provide a current control and indicating system, a difliculty arises due to the fact that the control must be effected at the high voltage end of the tube by controlling the voltage on the current control grid of the electron gun assembly. It is desired to efiFect current control by an individual situated at a control console located at a safe distance from the accelerator structure. Since the actual current control of the tube is effected by the control grid which is at a very high potential with respect to ground, a means must be provided for protecting the individual at the control console from the high voltages at the location where control must be actually effected. More particularly, it is desired to have the controls at the console operating at near ground potential so as to minimize dan er to the operator even though the actual control must be effected at a control grid which may be at a million or more volts with respect to ground.

A further difficulty in applying current control to the aforesaid apparatus is that the apparatus is housed in a pressurized container and in order to minmize leakage of the gaseous atmosphere from the container, it is desired to minimize the number and size of openings into the container in order to effect the desired control.

The difiiculties set forth above are overcome in accordance with the present invention by providing remote control of the voltage of the current control grid by means of a motor or other electromechanical transducer which operates at near ground potential. A mechanical insulating connection is maintained between the low and high voltage sections of the assembly so that the operator may provide control signals at near ground potential to effect operation of the motor or other electro-mechanical transducer and the coupling to the high voltage region is effected through a rod of insulating material, such as Lucite, connected to the motor shaft.

In accordance with another aspect of the present invention a motor control and/or current indicating circuit is provided which requires only a single opening into the pressurized housing. In one embodiment of the invention a two wire connection to the motor is employed to cause the motor to run in one direction or the other and a two wire connection back to the console is employed to indicate motor shaft position so that the operator may terminate operation of the motor when the shaft is at a position calculated to provide the desired beam current. The four wires are housed in a single small cable and require only one small opening into the pressurized container. In a second embodiment of the invention, a current measuring and motor rebalance circuit is employed to maintain the actual beam current at a preset beam current and only two wires are required to be connected between the console and the motor control and indicating circuit. In both cases the final angular position of the motor shaft is a :function of the desired beam current and this angular position is mechanically coupled to the high voltage end of the tube where it adjusts a mechanically controllable element (such as a potentiometer) to vary the input voltage to a control circuit for the current control grid of the beam tube. Thus, the only entry required into the pressurized housing is of a small number of relatively low voltage conductors. In one embodiment of the invention, the cable is a four-conductor cable and in the second embodiment of the invention, the conductor is only a two-conductor cable. Both of these cables, since they carry low voltages, may be quite small and it is a relatively simple matter to provide an adequate pressure seal about the cable where it passes into the chamber.

It is an object of the present invention to provide a current control apparatus for high-voltage, electron or ion beam accelerator tubes in which current through the tube is controlled by the voltage applied to a current control grid located at the high voltage end of the tube, and in which the voltage applied to the grid is under the control of an operator located at a control console operating at a low voltage; the coupling between the high and low voltage circuits being effected through a mechanical connection via an electro-mechanical converter such as a motor, relay, solenoid, etc.

It is another object of the present invention to provide for the current control of a high voltage ion or electron beam tube in which control at the high voltage end of the tube is effected by means of a mechanical connection to an electro-mechanical transducer located at the low voltage end of the tube which transducer is controlled from a control console located externally of the apparatus.

It is another object of the present invention to provide current control circuits for high voltage electron or ion beam accelerators in which a motor, located at a lower voltage end of the structure, is employed to adjust the voltage in a current control grid at the high voltage end of the structure and in which a relatively small number of wires must pass from the control console to the motor circuit located internally of a pressurized chamber surrounding the beam tube.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a partial electrical schematic and structural diagram of one embodiment of the apparatus of the present invention; and

FIGURE 2 is a partial electrical schematic and structural diagram of a second embodiment of the apparatus of the present invention.

Referring specifically to FIGURE 1 of the accompanying drawings, there is illustrated a small portion of an electron or ion beam accelerator tube 1 which is situated inside of a pressurized chamber 2. The beam tube 1 is provided with an electron gun section 3 comprising a heated cathode 4 and a current control grid 6. The tube illustrated is primarily for use as an electron beam accelerator tube since a heated cathode 4 is employed. However, it is to be understood that an ion source having an electrode for controlling ion current may be substituted for the heated cathode 4 so as to provide ions when it is desired to provide an accelerated ion beam. The instantaneous beam current through the accelerator tube 1 is determined by the voltage on the current control grid 6 and in accordance with the first embodiment of the present invention, there is provided an electronic feedback control circuit for controlling the voltage on the grid 6 in accordance with the setting of a potentiometer 7. More specifically, the potentiometer 7 is connected in series with a variable resistor 8 across a D.C. voltage source generally designated by the reference numeral 9 to provide a voltage divider circuit. For purposes of illustration, the voltage source comprises a transformer and D.C. rectifier assembly 11 supplied with alternating current across a pair of terminals 12. The voltage developed by the source 11 is applied across a pair of Zener diodes designated by reference numeral 13 to provide a Well regulated voltage across the series connected resistors 7 and 8.

The resistor 7 is provided with a slider 14 which is adjusted by a mechanical coupling 16 which may be a rotatable motor shaft or a translatable mechanical connection as will be described in more detail subsequently. The slider 14 associated with the resistor 7 is connected via a lead 17 to one side of the cathode 4 of the electron gun assembly 3 and connected between the high voltage source and the same side of the cathode 4 is a resistor 18; the resistor 18 being returned to ground through the high voltage source. In normal situations, the beam current flowing through the tube is collected at a target where it flows to ground. The circuit for the beam current therefore is from ground, through the high voltage source, resistor 18, cathode 4, the beam tube 1 and through the target back to ground. Thus, the voltage developed on the lead 17 is a direct linear function of the beam current flowing in the electron beam tube 1. This voltage is bucked against the portion of the voltage across the resistor 7 subsisting between the slider 14 and a further lead 19. In consequence, the voltage on the lead 19 is equal to the difference between the control voltage established between the lead 19 and slider 14 of resistor 7 and a voltage which is a direct function of the current through the beam tube and the voltage on the lead 19 constitutes the error voltage of the system.

The error voltage on the lead 19 is supplied to a high gain chopper stabilized D.C. amplifier 21 and thence to a driver amplifier 22 connected via an output lead 23- to the current control grid 6. If a difierence or an error voltage appears on the lead 19, indicating a difference between the actual current flowing through the resistor 18 and the desired current established by the position of the slider 14 on the resistor 7, a change in voltage is commanded and is applied to the current control grid 6 in such a sense as to reduce the error voltage on the lead 19 to zero.

The mechanical coupling 16 effects the setting of the slider 14 on the resistor 7 and therefore determines the current through the tube. The mechanical coupling 16 is driven by an AC. electric motor 24 having forward and reverse fields 26 and 27. The common point of the fields 26 and 27 is grounded and the other ends of the field are connected respectively to leads 28 and 29. These leads are coupled through a four-conductor cable 31 to a control console generally designated by the reference numeral 32. It will be noted that the motor 24 is disposed interiorly of the pressurized chamber 2, the four-conductor cable 31 exiting from the chamber through a pressure seal 33. Also disposed in the chamber 2, immediately adjacent the motor 24, are a pair of series connected resistors 34 and 36 with the resistor 34 having one end connected to a conductor 37 and the resistor 36 having one end connected to ground. The resistor 36 is provided with a slider 38 which is driven by the motor 24. The slider 36 is electrically connected to a further conductor 39 the conductors 37 and 39 forming the other two conductors of the cable 31.

The conductors 28 and 29 are connected at a control console, through push button switches 41 and 42, respec tively, to an AC. voltage source 43. The other end of the source 43 is grounded so that when one or the other of the push buttons 41 and 42 is depressed to close an associated switch, an energizing voltage is applied to one or the other of the leads 28 and 29 to cause the motor 24 to run in a forward or reverse direction depending upon which of the push buttons has been depressed.

The lead 39 is connected at the control console to one terminal of a null meter 44 While the lead 37 is connected to a positive terminal 46 of a D.C. source 47. Connected between the positive terminal 46 of the source 47 and ground are a pair of series connected resistors 48 and 49, the resistor 48 having one end connected to the terminal 46 and the resistor 49 having one end connected to ground. The resistor 49 is provided with a slider 51 connected via a lead 52 to the other terminal of the null meter 44. A calibrated dial 53 may be mechanically coupled to the slider 51 so as to move the slider and adjust the voltage on the lead 52. The dial 53 may be calibrated in units of beam current.

The resistor 48 and resistor 49 with its associated slider 51 in conjunction with the resistor 34 and resistor 36 with its associated slider 38 form a Wheatstone bridge with the null meter 44 connected between its conjugate terminals. When the bridge is balanced the null meter 44 provides a null indication of this condition, also indicating that the voltage position of the slider 38 corre-- sponds to the voltage position of the slider 51.

In operation, when it is desired to establish a specific current through the beam tube 1, the knob 53 is operated so that the conventional pointer on the knob is disposed adjacent the calibration indicating the desired beam current. One of the push buttons 41 or 42, depending upon the desired direction of rotation, is then actuated and the motor is rotated and moves the slider 38. When the null meter 44 indicates zero or null, the operation of the motor is terminated and as is apparent, the position of the motor shaft is a function of the desired setting of beam current. The shaft 16 upon rotation by the motor by a sufiicient amount to rebalance the bridge, adjusts the slider 14 on the resistor 7 to a new setting indicative of the desired current; that is, indicative of the setting of the slider 51 on the resistor 49. A voltage now appears on the lead 19 indicative of the difference between the desired and actual beam current and via amplifiers 21 and 22 causes the voltage on the current control grid 6 to be adjusted until the voltage on the lead 19 becomes zero.

In the apparatus of FIGURE 1, the mechanical linkage 16 must comprise a non-conductive material and an example of a material which is completely suitable for such an application is Lucite. The Lucite rod .16 may be directly connected to the motor shaft so that no gears are involved and therefore no mechanical errors are introduced into the system. The motor 24 may be replaced with any suitable type of electro-mechanical transducer and may comprise a solenoid which couples a translatory motion to the shaft 16 for driving the slider 14. Other suitable types of electro-mechanical transducers are Well known in the art and are adequate for the intended purpose. It has been found with the apparatus of FIGURE 1, that by employing slide wire potentiometers having a linearity of 0.1%, control of current over a range of 50 microamperes to milliamperes may be maintained to within :0.1%.

It may be desirable in certain instances to measure the current supplied to a target rather than to measure the total current supplied to the tube at the cathode. In the operation of the linear accelerator, the current introduced at the cathode is not necessarily the same as the current that is delivered to the target which is being irradiated. In the second embodiment of the invention illustrated in FIGURE 2, there is provided an apparatus for controlling and measuring the current actually supplied to the accelerator target. In such a system the object to be irradiated is connected via a collector electrode to ground through a low value resistor 56. The ungrounded end of the resistor 56 is connected via a lead 57 to a slider 58 operable over a resistor 59. The target in such applications is located outside of the pressurized chamber 2 and therefore there is no difficulty in bringing leads through the chamber wall to a control console. The resistor 59 is connected across a source of DC voltage 61 which may be a battery, as illustrated, or a rectified voltage derived from an A.C. line. The negative end of the source 61, is connected to a suitable A.C. amplifier 62 also located at the control console. The reversible phase output voltage is developed on a lead 60 and is applied to a two phase A.C. motor 63 via a cable 64 which passes into the interior of the chamber 2. The motor 63 has a second field supplied with a reference phase over a further lead 65 of the cable 64.

The shaft of the motor 63 is coupled via a suitable mechanical link, such as a Lucite rod, to a slider 68 operable over a resistor 69. The resistor 69 is connected in series circuit with a further resistor 71 across a source of DC. potential 72. An amplifier 73 is connected between the slider 68 and one end of the resistor 69 so that the voltage applied to the amplifier 73 is a function of the position of the slider 68 on the resistor 69. The output voltage developed by the amplifier 73 is applied via a lead 74 and further amplifiers, if desired, to the current control grid of an electron or ion beam tube.

In operation, the voltage developed between the slider 58 and the negative end of the resistor 59 is in opposition to the voltage developed across the resistor 56. When the current to the target is equal to the current called for by the position of the slider 58, zero voltage is developed at the input circuit of the amplifier 62. If for any reason the current to the target deviates and an error voltage is developed at the input circuit to the amplifier 62, the motor 63 is energized in such a sense as to reposition the slider 68 to increase or decrease the current to the load to balance the system. When it is desired to vary the current to a load in response to a command, the slider 58 is positioned by the operator so as to call for a difference in load current and again a voltage indicative of this change is applied to the input circuit of the amplifier 62. The motor 63 is energized and adjusts the voltage on the current control grid of the beam tube until the system is again balanced. The slider 53 may employ a calibrated knob for adjustment of the slider 58.

It is to be understood that in both embodiments of the present invention, appropriate frequency sensitive networks may have to be employed to maintain the closedloop stability of the system.

While I have described and illustrated several specific embodiments of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. A current controlsystem for a high-voltage, beam tube accelerator having a current control electrode located at the high voltage end of the tube, said current control system comprising means for establishing a first voltage, indicative of a desired beam current, an electromechanical transducer having a mechanical output element, means for energizing said transducer, such that said mechanical output element assumes a position which is a function of said first voltage, a variable voltage means, means for varying said variable voltage means to produce a second voltage indicative of the position of said mechanical output element and means for applying a voltage to the current control electrode of the beam tube which is a function of said second voltage.

2. A current control system for a high-voltage, beam tube accelerator having a current control electrode located at the high voltage end of the tube, said current control system comprising means for establishing a first voltage relative to a low voltage reference potential indicative of a desired beam current, an electro-mechanical transducer having a mechanical output element, means for energizing said transducer such that said mechanical output element assumes a position which is a function of said first voltage, a variable voltage means for producing a second voltage variable with respect to a high voltage reference potential, means for coupling said mechanical output element to said variable voltage means so that the value of said second voltage relative to said high voltage reference potential is indicative of the position of said mechanical output element and means for applying a voltage to the current control electrode of the beam tube which is a function of said second voltage.

3. A current control system for a high-voltage, beam tube accelerator having a high-voltage applied to the current generating end of the beam tube and the other end of the tube operating at near ground potential, the beam tube accelerator having a current control electrode disposed at the high-voltage end of the beam tube, said current control system comprising means for establishing a first voltage, relative to ground potential, indicative of a desired beam current, an electromechanical transducer located adjacent the low-voltage end of the beam tube and having a mechanical output element, means for causing said mechanical output element of said transducer to assume a position indicative of said first voltage, a variable voltage means located adjacent the highvoltage end of the beam tube, an insulating mechanical connection between said mechanical output element and said variable voltage source such that said variable voltage source produces a second voltage relative to said high-voltage which is a function of the position of said mechanical output element, and means for applying said second voltage to said current control electrode.

4. The combination according to claim 3 further comprising means for establishing a third voltage indicative of beam current, and wherein means for applying includes means for developing an error voltage determined by the difierence between said second and third voltages, and means for applying a voltage to the current control electrode such as to maintain said error voltage at a minimum.

5. The combination according to claim 3 further comprising means for establishing a third voltage indicative of beam current at the low voltage end of the beam tube, and wherein said means for energizing comprises means for developing an error voltage indicative of the difference between said first and third voltages, said error voltage bein applied to said transducer in such a sense as to minimize the error voltage.

6. The combination according to claim 3 wherein said beam tube accelerator is situated in a pressurized container, said means for establishing a first voltage and said means for causing being located essentially externally of said container, said transducer being located internally of said container, and a multiconductor cable extending between said means for establishing and said means for causing on the one hand and said transducer on the other.

7. The combination according to claim 3 wherein said accelerator is located in a pressurized chamber and wherein said means for establishing comprises a first variable voltage divider having one end grounded and located externally of said chamber, and wherein said means for causing comprises switch means located externally of said chamber and a second variable voltage divider tainer, said voltage divider comprising a bridge circuit, a null indicator, said transducer being connected to vary the second voltage divider, a cable having four conductors extending into said chamber, one of said conductors connecting the ungrounded ends of said voltage dividers, a second of said conductors connecting a terminal of said null meter to a conjugate terminal of said bridge and the two other of said conductors connecting said transducer to said switches to produce reversible operation of said transducer.

No references cited.

ARTHUR GAUSS, Primary Examiner.

having one end grounded located internally of said con- 15 EPSTEIN, Assistant Examiner- 

1. A CURRENT CONTROL SYSTEM FOR A HIGH-VOLTAGE, BEAM TUBE ACCELERATOR HAVING A CURRENT CONTROL ELECTRODE LOCATED AT THE HIGH VOLTAGE END OF THE TUBE, SAID CURRENT CONTROL SYSTEM COMPRISING MEANS FOR ESTABLISHING A FIRST VOLTAGE, INDICATIVE OF A DESIRED BEAM CURRENT, AN ELECTROMECHANICAL TRANSDUCER HAVING A MECHANICAL OUTPUT ELEMENT, MEANS FOR ENERGIZING SAID TRANSDUCER, SUCH THAT SAID MECHANICAL OUTPUT ELEMENT ASSUMES A POSITION WHICH IS A FUNCTION OF SAID FIRST VOLTAGE, A VARIABLE VOLTAGE MEANS, MEANS FOR VARYING SAID VARIABLE VOLTAGE MEANS TO PRODUCE A SECOND VOLTAGE INDICATIVE OF THE POSITION OF SAID MECHANICAL OUTPUT ELEMENT AND MEANS FOR APPLYING A VOLTAGE TO THE CURRENT CONTROL ELECTRODE OF THE BEAM TUBE WHICH IS A FUNCTION OF SAID SECOND VOLTAGE. 